From German utility model DE-GM No. 74 00 309 there is known an electric cartridge heater in which a casing accommodates a carrier block of insulating material which supports a heating conductor helically wound around the carrier block. Such a cartridge heater is tightly inserted into a bore of a device to be heated so that the heat generated by the cartridge heater is transmitted to the device almost without any loss.
The cartridge heater according to the prior art has, however, the disadvantage that there is no provision to permit excess heat to be carried away.
It is, however, known to equip such cartridge heaters with thermostats which turn off the heater once the device has reached the desired temperature. In case the temperature of the device is below the desired value, the thermostat activates the cartridge heater so that the desired temperature can be kept approximately at a constant value.
Although this principle may seem to be sound, there is the drawback that in case additional heating sources are present, e.g. generating frictional heat, a temporary overheating may be obtained when the device is already at its desired temperature. Such overheating cannot be corrected by the thermostat since it acts only when the temperature of the device is below a desired value. Thus a temporary overheating of the device and of the cartridge heater cannot be prevented.